[Gutenberg Starter Theme] Try pulling starter content in from an external file

[kjellr]

This PR tries out an idea that came up during a discussion with @epiqueras yesterday — that starter content could theoretically be pulled in from a standalone "block template"-style file, rather than being declared as a complicated array in PHP, as is done in themes like this repo's version of Twenty Twenty.

This PR creates a new block-starter-content folder for the Gutenberg Starter Theme (Blocks). It contains a post-demo.html file, full of the content from Gutenberg's demo post. This content gets pulled into the homepage via file_get_contents() for now. I'm not sure if that's the right function, or if it needs to be escaped and/or limited in some way... I could see this being problematic if the file size were huge. But it appears to work for this example at least. I'd love some more experienced PHP 👀 on that method.

To test:

1. Start a brand new WordPress site.
2. Clone this repository or upload a zip of the Gutenberg Starter Theme (Blocks) into your site’s themes directory.
3. Install the Gutenberg plugin.
4. Enable the "Full Site Editing" experiment in the Gutenberg > Experiments panel.
5. Activate the new theme from within Appearance > Themes.
6. Visit the Customizer to automatically load the demo content.

The homepage should now contain the demo content from block-starter-content/post-demo.html.

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Screenshot

[jffng]

👏 This is an interesting idea!

What it gets around is the concept that block templates and template parts are only intended for re-usable parts of the site (i.e. not for demo content). Previously I had tried populating starter content with block templates, even though this mixed the idea of template and content in a potentially confusing way. In my opinion, explicitly having a block-starter-content folder helps keep the distinction clear conceptually.

I can't comment on the file_get_contents method or the robustness of starter content as a bulletproof mechanism to populate a fresh site (I did run into some starter content issues when I placed the demo content for TwentyTwenty). I might try stress testing this by including many posts and large files.

[jffng]

@kjellr shall we close this? Seems like content like this, if at all, could ship with a pattern.

[kjellr]

Yeah, sounds good to me. 👍